A low dropout (LDO) regulator can be used to provide regulated power to an electronic device at a level close to a level of a power source provided to the LDO regulator. FIG. 1 shows an exemplary case where an LDO regulator (110) is used to provide power to a radio frequency (RF) power amplifier (150) based on a power source Vbatt. The LDO regulator (110) comprises an operational amplifier (115) whose output is connected to a current driver circuit (120) capable of handling current high enough for operation of the RF power amplifier (150) at a desired output voltage of the LDO regulator, LDO_out. Such output voltage, LDO_out, is sampled through, for example, a voltage division network formed by resistors (R1, R2) and fed back to the positive input terminal of the operational amplifier (115), to create a closed loop control of the output voltage LDO_out such as to track an input voltage fed to the inverting input terminal of the operational amplifier (OpAmp) (115). In the described closed loop mode of operation, the LDO regulator (110) regulates the output voltage LDO_out to remain substantially constant irrespective of the current required by the RF power amplifier during operation.
With further reference to FIG. 1, the current driver circuit (120) can comprise one or more transistor devices, such as P-type MOSFETs (PMOS), which during regulation of the output voltage, LDO_out, operate in their respective saturation regions of operation and can therefore provide a corresponding high enough gain for operation of the above described closed loop control.
As known to a person skilled in the art, transistors of the current driver circuit (120), also referred to as “pass transistors”, or “pass devices”, can contribute to a power loss, which in some cases may be a disadvantage. Accordingly, it can be desirable to minimize such power loss in the pass devices, which can be achieved by driving the pass devices into their respective triode (linear) regions of operation by applying a corresponding biasing voltage to the gates of the pass devices. When driven into their respective triode regions of operation, the pass devices have a very low gain which can cause the LDO to operate in essentially an open loop mode where the input voltage is not tracked and therefore the LDO_out voltage is not regulated. Due to the capacitive loading presented by the pass device (101) connected to the output of the OpAmp (115), driving the pass device (101) back into its saturation region of operation can be delayed by a time it takes the output of the OpAmp (115) to discharge the capacitive load. It can be desirable to reduce such delay while maintaining a low power operation of the OpAmp (115) as described in the following paragraphs.